This invention relates to a method of fabricating window panels, particularly curved panels used in aircraft. Such panels usually comprise a rigid metal mounting frame surrounding a transparency. The transparency is usually a laminated window and comprises at least two rigid sheets of transparent material selected from the group consisting of glass, polycarbonate, acrylic resins and hard polyurethanes, (which will be referred to as "glass" in order to simplify this specification) and a layer of thermoplastic interlayer material bonding adjacent of said rigid sheets together. The thermoplastic interlayer material may be composed of a plasticized polyvinyl acetal such as polyvinyl butyral or it may be composed of a polyurethane and, regardless of its composition, will be referred to as an "interlayer". A metal reinforcing insert has an interior portion embedded within the thickness of an interlayer at a predetermined distance in the direction of the thickness of said transparency from the outboard surface thereof. The metal reinforcing insert (usually an aluminum frame) extends outward into an outer portion that is attached to the mounting frame. Usually, the outer portion of the reinforcing insert and the outer metal mounting frame have aligned apertures which are adapted to receive attachment bolts that attach the panel to an aircraft body.
It is traditional in the fabrication of aircraft windshields to assemble the components of the transparency which includes two or more sheets of glass with intervening interlayers of flexible and compressive material interposed between adjacent glass layer. The interlayers comprise a plurality of plies. In one of the interlayers used to bond adjacent glass sheets of different outline configurations, the plies of interlayer material adjacent one of said adjacent glass sheets conform in outline to that of said one glass sheet and the plies of interlayer adjacent the other said adjacent glass sheets conform in outline to that of the other adjacent glass sheet. If a reinforcing insert is embedded between the outer portions of the adjacent glass sheets, plies of the interlayer whose perimeter conforms to the inner perimeter edge of the reinforcing insert are disposed in alignment with the thickness of the insert and intermediate the other plies of the interlayer. When an assembly containing such a reinforcing insert disposed within the thickness of one of the interlayers is subjected to heat and pressure for lamination, the flexibility of the interlayer plies permits the reinforcing insert to move, tilt and deflect somewhat so that its different portions move in the direction of the thickness of the interlayer for different distances during lamination. Therefore, the laminated transparency which contains an extended outer portion of the reinforcing insert, which outer portion was later attached to the metal mounting frame according to the method performed prior to the present invention, was not at a uniformly equal distance throughout its extent with the outer surface of the transparency. Consequently, when the laminated transparency had the outer portion of its reinforcing insert secured to the mounting frame, stresses were established.
Some of the results of these stresses during use of an aircraft containing panels produced in the manner of the prior art included failure of the seal between the transparency and its mounting frame. This allowed moisture to penetrate between the outer portion of the reinforcing insert and the mounting frame. It also caused delamination of the elements of the transparency which are subject to weakening of the bond therebetween when subjected to moisture. Furthermore, the inner surface of the outboard sheet is usually provided with a transparent electroconductive coating and a pair of bus bars. Solder joints between the bus bar and lead wires can become blackened and discolored which indicates degradation of the solder joints due to the moisture penetration. Also, moisture tends to attack the electroconductive coating as is evidenced by increased resistance between bus bars and eventual electrical arcing. Another problem found in some aircraft panels made according to the prior art method described previously is that the resulting panels have less structural rigidity than is required to avoid a tendency for the panels to permit moisture to penetrate and enhance delamination of the panel elements.
An examination of a laminated windshield inspected after field use indicated that the reinforcing insert was in a plane non-parallel to the outer surface of the laminated transparency. This non-parallelism of the reinforcing insert relative to the outer glass surface was both parallel to the length and across the width of the reinforcing insert.
A proposed solution for the above problem involved making the mounting frame and the metal insert as a unitary structure. The cost of milling such a complicated structure is so great as to render this proposal impractical.
A different method of fabricating window panels comprising a metal mounting frame enclosing a transparency having a metal reinforcing insert extending from an inner portion within an interlayer thereof to an outer portion that is attached to the mounting frame was believed necessary to avoid the problems that existed prior to the present invention.